Generally, although a signal processing is carried out in a way of differential inputs and differential outputs, the final output signal should be in the form of single-ended output signal in order to achieve a high dynamic range in case of such as audio analog signals.
Further, a differential input differential output operational amplifier (also referred to as a fully differential operational amplifier) is generally used to improve the dynamic range and the power supply rejection ratio ("PSRR") of the sigma-delta (".SIGMA.") digital-to-analog converter ("DAC"), such as audio and voice DACs, to which an oversampling method is applied. In this case, however, the single-ended output converting circuit is also needed to provide the single final output. Moreover, this differential input single-ended output generally has an additional function of low pass filtering.
FIG. 1 shows one example of conventional differential input single-ended output analog low pass filter used in the audio DAC of Burr-Brown company. As shown in the drawing, 6 passive resistors R1 to R6 and four (4) capacitors C1 to C4 are organized to function as a secondary low pass filter.
However, the conventional differential input single-ended output analog low pass filter needs a large chip area when implemented on a semiconductor chip. Further, in case the pole of the filter moves to low frequency range, the RC time constant of the filter should be made larger than that of high frequency range case. As a result, the needed chip area will be much larger. As described above, the large chip area of the conventional filter results from the number of the passive resistors and the capacitors.